Maybe for these types of studies you could try to focus on improving one thing in particular, like pushing the gesture, proportion, mannequinization, specific parts of anatomy, etc. If it’s a quick sketch study it shouldn’t take as long as something like a project. I think focusing on specific things you’d like to improve on and having a goal could help you find direction in these studies. Hope this helps!

I guess a study should be a quick drawing in the direction you're aiming. I really like your mannequins, although I'm not sure you have the angle of the chest quite right in the supine one. I think when you worked on the fleshy ones you lost some of the big areas of mass from your mannequins a bit, and maybe the proportions aren't quite right, but I think it's just down to practice. I would maybe try a few versions from each reference. A quick gesture, a mannequin and then a fleshy one. Figure out the differences, see what you like, and then a final drawing with all the bits you like together.